Generally, pure water flux characteristics of most reverse osmosis membranes can be greatly enhanced, without commensurate decrease in impurity rejection capability, by reducing the membrane thickness from the standard 5 to 15 mils, to the ultra-thin 0.01 to 1 mil range. To date, use of ultra-thin membranes, especially those having a substantially uniform cross section; has been hampered by the inability to handle these symmetric membrane films after casting. The glass casting plates usually employed as the film casting surface are smooth, heat and solvent resistant, and provide a suitable wettable surface with the solvents generally used; however, the ultra-thin membranes adhere to the plates to such an extent that it is virtually impossible to remove the membranes without damage. It is even more difficult to insert the ultra-thin membranes into a liquid purification system without harming them in the process.
Riley, in U.S. Pat. No. 3,648,845, formed 0.004 mil thick cellulose acetate membranes, in situ, on a porous glass, metal or ceramic support. The support had its pores covered by a polyacrylic acid barrier layer which was disposed between the membrane and the support, and which could be partially dissolved away. This provided a direct method of making a cellulose acetate membrane-solid support system, which avoided the ultra-thin membrane handling problem. A method to solve the handling problem especially for symmetric reverse osmosis membranes is needed.